Studies are being conducted on the biosynthesis of thyroxine through an investigation of the action of purified thyroid peroxidase on the iodination and coupling of tyrosine-containing oligopeptides. Parallel studies are also being performed with lactoperoxidase and horse radish peroxidase for purposes of comparison. In addition, the action of intracellular tissue proteinases such as cathepsin D, as well as other acid proteinases, on protein and oligopeptide substrates is under investigation, in relation to the release of diiodotyrosine and thyroxine from thyroglobulin. BIBLIOGRAPHIC REFERENCE: Kinetics of action of pepsin on fluorescent peptide substrates. G. P. Sachdev and J. S. Fruton. Proc. Nat. Acad. Sci. 72, 3424-3427 (1975). The specificity of proteinases toward protein substrates. J.S. Fruton. In Proteases and Biological Control (E. Reich, D.B. Rifkin and E. Shaw, Eds.) Cold Spring Harbor Laboratory, 1975, pp. 33-50.